Applications of diffusion hardening techniques

ABSTRACT

A device, for example a medical implant, and a method of making the same, the device having a metal or metal alloy substrate, for example CoCr, and a diffusion hardened metallic surface, for example a plasma carburized surface, contacting a non-diffusion hardened surface or a diffusion hardened surface having a diffusion hardening species different from that of the opposing surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/US2007/060719, filed Jan. 7, 2007, which claims priority to U.S.provisional patent application Ser. No. 60/759,843 filed on Jan. 18,2006. Each prior application is incorporated by reference in itsentirety as though fully disclosed herein.

TECHNICAL FIELD

This invention pertains to devices having metallic surfaces whichcontact other metallic surfaces. In preferred embodiments, the inventionpertains to a hard-on-hard medical implant. Preferably, the inventionpertains to a medical implant having a metal or metal alloy substrateand a carburized surface articulating against a non-carburized surface.

BACKGROUND OF THE INVENTION

Medical implant materials, in particular orthopedic implant materials,must combine high strength, corrosion resistance and tissuecompatibility. The longevity of the implant is of prime importanceespecially if the recipient of the implant is relatively young becauseit is desirable that the implant function for the entire lifetime of apatient. Because certain metal alloys have the required mechanicalstrength and biocompatibility, they are ideal candidates for thefabrication of prostheses. These alloys include 316L stainless steel,chrome-cobalt-molybdenum alloys (CoCr), titanium alloys and morerecently zirconium alloys which have proven to be the most suitablematerials for the fabrication of load-bearing and non-load bearingprostheses.

In a traditional hip implant, CoCr alloy femoral head articulatesagainst polyethylene. Owing to the relative hardness of each of thearticulating components, this type of implant is frequently referred toin the art as a hard-on-soft implant; the CoCr alloy being the hardcomponent of the articulating couple, and the polyethylene being thecorresponding soft component. During the life of the implant, wear ofthe polyethylene exceeds that of the hard component. The polyethylenewear debris in turn may lead to osteolysis and loosening of the implant.A considerable amount of effort has been expended in the prior art in aneffort to reduce this wear. One of the approaches has been to change thecharacteristics of polyethylene to improve its wear characteristics, forexample by cross-linking. Another approach has been to change thecharacteristics of the femoral head. One such approach has been taughtby Davidson (U.S. Pat. No. 5,037,438). Davidson recommends use of anoxidized zirconium surface to reduce the wear of polyethylene. Eventhough such approaches have led to significant reduction in wear ofpolyethylene, there has been a growing demand for much more wearresistant implants. This need comes from young and active patients whowant to return to their normal lives after the joint replacement.Another requirement of these young and active patients is the jointstability. Typically, a larger-anatomical joint is more stable than asmaller joint. Polyethylene due to its lower strength can not be madebeyond certain sizes and thus limits its use for young and activepatients. This has led to emergence of hard-on-hard metal implants.

Currently, there are two primary types of hard-on-hard hip implants thatare available commercially. These are metal-on-metal (including metalalloy-on-metal alloy) and ceramic-on-ceramic. The current standardmaterial of metal-on-metal implants is high carbon CoCr alloy. The majorconcern with the metal-on-metal implant is the metal ion release fromthe joint and its unknown effects on the physiology of the human body.The advantage of metal-on-metal implants is that they can be used inlarger sizes. The larger size of the implant allows greater range ofmotion and can provide more joint stability. The metal-on-metal implantshave also been shown to be useful for resurfacing type of applicationwhere conservation of bone is desired. In such larger joints, theconventional or cross-linked polyethylene is not preferred andmetal-on-metal may be the only choice available. The larger sizerequires polyethylene liner to be thinner. A thinner liner may not bemechanically strong, may creep more or may lead to increased wear andosteolysis and eventually failure of the implant. In general, the classof hard-on-hard implants can be significantly broadened. It can includecomponents articulating against each other that are made from metals orceramics or any combinations thereof. In this disclosure, the term hardrefers to metals and or ceramics. Thus “hard-on-hard” can bemetal-on-metal metal-on-ceramic, and ceramic-on-ceramic. In theforegoing context, “metal” includes both pure metals and metal alloys.

The other commonly used hard-on-hard joint is ceramic-on-ceramic. Thecurrent standard material of ceramic-on-ceramic implants is alumina.Metal ion release is typically not a concern for these implants. But dueto limited toughness and brittle nature of ceramics, it is difficult tomake these implants in larger sizes. The ceramic components have finiteprobability of fracture thus leading to a potential joint failure andcomplications associated with the fracture of a joint.

It has been an object of much of the prior art to reduce the metal ionrelease and minimize the fracture risk by combining metal and ceramiccomponents. Fisher et al. (U.S. Patent Application 2005/0033442) andKhandkar et al. (U.S. Pat. No. 6,881,229) teach the use of an implanthaving a metal-on-ceramic articulation. Fisher et al teach that thedifference in hardness between the metallic component and the ceramiccomponent to be at least 4000 MPa. Khandkar et. al. specifically teachuse of silicon nitride ceramic components for articulating against themetallic component. In both instances, the objective is to lower thewear of mating couples. But in both instances, the fracture risk ofceramic is still significant. In both instances, the strength of ceramiccomponent influences how large the joint size can be made. It should benoted that in both instances it is the ceramic surface that is matingwith a metallic surface. As discussed in the details below, the objectof this invention is to reduce the wear of the mating couples when bothcomponent surfaces are metallic in nature. In another approach,Lippincott and Medley (U.S. Pat. No. 6,059,830) teach applyinggeometrical constraints to the mating hip components. The '830 patentteaches the use of components such that the radius difference of themating components is less than 50 microns. This small difference inradius will promote thicker fluid film formation and thus reduced wearof mating metallic components. The disadvantage of this method is that asophisticated manufacturing set-up is required to produce componentswith such tight tolerances.

The problems relating to a low wear hard-on-hard metallic couple is notunique to the medical implant field, and exists in other fields of artas well, examples of which include automotive and aerospace components.Other bearing applications would also benefit from an improved couple.

The inventors of the present invention have found that such a demandingmanufacturing approach is not necessary because significant improvementsin wear reduction can be realized through the differential treatment ofone surface of two contacting surfaces, and in particular where thecontacting surfaces articulate against one another.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a medical implant, and a method ofmaking the same, having a metal or metal alloy substrate, for exampleCoCr, and a diffusion hardened, surface, for example a plasma carburizedsurface, contacting a non-diffusion hardened surface or a diffusionhardened surface having a diffusion hardening species different fromthat of the opposing surface.

In one embodiment, there is device comprising: a first portion, saidfirst portion having a first contacting surface; a second portion, saidsecond portion having a second contacting surface; wherein the firstportion and the second portion are comprised of metal or metal alloy,and wherein the first and second contacting surfaces contact oneanother; and, wherein one of conditions (a) and (b) is met: (a) one ofsaid first and second contacting surfaces comprises a diffusion hardenedmetallic surface which is diffusion hardened with a diffusion hardeningspecies and the other of said first and second contacting surfaces is ametallic surface that is not diffusion hardened; (b) both of said firstand second contacting surfaces comprise a diffusion hardened metallicsurface wherein said first contacting surface is diffusion hardened withat least a first diffusion hardening species and said second contactingsurface is diffusion hardened with at least a second diffusion hardeningspecies and wherein said first diffusion hardening species is differentfrom said second diffusion hardening species; wherein said diffusionhardening species is selected from the group consisting of carbon,nitrogen, oxygen, boron, and any combination thereof. In anotherembodiment, the device is a medical implant. In some embodiments, themetal or metal alloy comprising said first portion is different from themetal or metal alloy comprising said second portion. In some embodimentsof the device, the metal or metal alloy is CoCr. In some embodiments ofthe device, one or both of said contacting surfaces comprises a plasmahardened surface. In some embodiments of the device, the plasma hardenedsurface is selected from the group consisting of a plasma carburizedsurface, a plasma nitrided surface, a plasma oxidized surface, a plasmaborided surface and any combination thereof. In some embodiments whereinthe device is a medical implant, the medical implant is a hip implant.In some hip implant embodiments, one of said first portion and secondportion is a femoral component comprising a femoral head and whereinsaid femoral head comprises a contacting surface; and wherein the otherof said first portion and second portion is an acetabular component,said acetabular component comprising a inner surface wherein said innersurface comprises a contacting surface. In some hip implant embodiments,one or both of said contacting surfaces comprises a plasma hardenedsurface. In some hip implant embodiments, said plasma hardened surfaceis selected from the group consisting of a plasma carburized surface, aplasma nitrided surface, a plasma oxidized surface, a plasma boridedsurface and any combination thereof. In some medical implantembodiments, the medical implant is a knee implant. In some knee implantembodiments, one of said first portion and second portion is a femoralcomponent comprising at least one condyle, said at least one condylecomprising a contacting surface, and wherein the other of said firstportion and second portion is a tibial component, said tibial componentcomprising a contacting surface. In some knee implant embodiments, oneor both of said contacting surfaces comprises a plasma hardened surface.In some knee implant embodiments, said plasma hardened surface isselected from the group consisting of a plasma carburized surface, aplasma nitrided surface, a plasma oxidized surface, a plasma boridedsurface and any combination thereof. In some medical implantembodiments, the medical implant is selected from the group consistingof a shoulder implant, an elbow implant, a finger implant, a vertebralimplant, and a tempro-mandibular implant. In some embodiments of thedevice, said first contacting surface and said second contacting surfacearticulate against one another. In some embodiments of the device, thedevice further comprises a deposited ceramic coating on one or both ofsaid first and second contacting surfaces. In some embodiments of thedevice comprising a deposited ceramic coating, said deposited ceramiccoating is selected from the group consisting of ceramic oxides, ceramicnitrides, ceramic carbides, ceramic borides, and any combinationthereof. In some embodiments of the device, the device further comprisesa diamond or diamond-like carbon coating on one or both of said firstand second contacting surfaces. In some embodiments of the device, bothof said first and second contacting surfaces comprise a diffusionhardened metallic surface and wherein said first contacting surface isdiffusion carburized and diffusion oxidized, and said second contactingsurface is diffusion carburized. In some embodiments of the device,condition (b) applies and, said diffusion hardening species for saidfirst contacting surface is selected from the group consisting ofcarbon, nitrogen, boron, and any combination thereof; and, saiddiffusion hardening species for said second contacting surface isselected from the group consisting of carbon, nitrogen, oxygen, boron,and any combination thereof, subject to the condition that one of saidcontacting surfaces is diffusion hardened with a diffusion hardeningspecies that is not present as a diffusion hardening species in theother contacting surface. In some embodiments of the device, saiddiffusion hardening species is selected from the group consisting ofcarbon, nitrogen, oxygen, and any combination thereof. In someembodiments of the device, the device is an automotive or aerospacedevice.

In another embodiment, there is a method of making a device comprisingthe steps of: forming a device from a metal or metal alloy, said devicecomprising a first portion, said first portion having a first contactingsurface, and a second portion, said second portion having a secondcontacting surface, wherein said first and second contacting surfacescontact one another; forming a diffusion hardened metallic surface onsaid device by diffusion hardening, said step of diffusion hardeningbeing performed with a diffusion hardening species selected from thegroup consisting of carbon, nitrogen, oxygen, boron, and any combinationthereof, said step of diffusion hardening performed according to step(a) or (b): (a) diffusion hardening to form a diffusion hardenedmetallic surface on one and only one of said first and second contactingsurfaces; (b) diffusion hardening to form a diffusion hardened metallicsurface on both of said first and second contacting surfaces whereinsaid first contacting surface is diffusion hardened with at least afirst diffusion hardening species and said second contacting surface isdiffusion hardened with at least a second diffusion hardening speciesand wherein said first diffusion hardening species is different fromsaid second diffusion hardening species. In some embodiments, the stepof forming a device comprises forming a medical implant. In someembodiments, the step of forming a device from a metal or metal alloycomprising a first portion and a second portion comprises forming saidfirst portion and said second portion from different metal or metalalloys. In some embodiments, the metal or metal alloy is CoCr. In someembodiments, the said step of diffusion hardening comprises plasmahardening. In some embodiments wherein a medical implant is formed, saidstep of forming a medical implant comprises forming a hip implant. Insome embodiments wherein a hip is formed, one of said first portion andsaid second portion is an acetabular component and the other of saidfirst portion and said second portion is a femoral component. In someembodiments wherein a medical implant is formed, said step of formingcomprises forming a knee implant. In some embodiments wherein a kneeimplant is formed, one of said first portion and said second portion isa tibial component and the other of said first portion and said secondportion is a femoral component. In some embodiments wherein a medicalimplant is formed, said step of forming a medical implant comprisesforming a medical implant selected from the group consisting of ashoulder implant, an elbow implant, a finger implant, a vertebralimplant, and a tempro-mandibular implant. In some embodiments, themethod further comprises the step of depositing a ceramic coating on oneor both of said first and second contacting surfaces. In someembodiments, the method further comprises the step of depositing adiamond or diamond-like carbon coating on one or both of said first andsecond contacting surfaces. In some embodiments, the step of diffusionhardening to form a diffusion hardened metallic surface on both of saidfirst and second contacting surfaces comprises forming a diffusioncarburized and diffusion oxidized surface on said first contactingsurface and forming a diffusion carburized surface on said secondcontacting surface. In some embodiments wherein step (b) applies, saidstep of diffusion hardening for said first contacting surface isperformed with a diffusion hardening species selected from the groupconsisting of carbon, nitrogen, boron, and any combination thereof; and,said step of diffusion hardening for said second contacting surface isperformed with a diffusion hardening species selected from the groupconsisting of carbon, nitrogen, oxygen, boron, and any combinationthereof, subject to the condition that one of said contacting surfacesis diffusion hardened with a diffusion hardening species that is notpresent as a diffusion hardening species in the other contactingsurface. In some embodiments, said step of diffusion hardening to form adiffusion hardened metallic surface is performed with a diffusionhardening species selected from the group consisting of carbon,nitrogen, oxygen, and any combination thereof.

In another embodiment, there is a medical implant comprising: a firstportion, said first portion having a first contacting surface; a secondportion, said second portion having a second contacting surface; whereinthe first portion and the second portion are comprised of metal or metalalloy, and wherein the first and second contacting surfaces contact oneanother; and, wherein one of conditions (a) and (b) is met: (a) one ofsaid first and second contacting surfaces comprises a diffusion hardenedsurface and the other of said first and second contacting surfaces isnot diffusion hardened; (b) both of said first and second contactingsurfaces comprise a diffusion hardened surface wherein said firstcontacting surface is diffusion hardened with a first diffusionhardening species and said second contacting surface is diffusionhardened with a second diffusion hardening species and wherein saidfirst diffusion hardening species is different from said seconddiffusion hardening species; wherein the diffusion hardened surfaces in(a) and (b) are selected from the group consisting of diffusioncarburized surfaces and diffusion nitrided surfaces. In someembodiments, the metal or metal alloy comprising said first portion isdifferent from said metal or metal alloy comprising said second portion.In particular embodiments, the metal or metal alloy is CoCr. Inparticular embodiments, the diffusion hardened surface is a plasmahardened surface. In some embodiments, the plasma hardened surface isselected from the group consisting of a plasma carburized surface, aplasma nitrided surface and a surface both plasma carburized and plasmanitrided. In some embodiments, the medical is a hip implant. Inparticular embodiments of a hip implant, the first portion is a femoralcomponent comprising a stem and a femoral head and wherein said femoralhead comprises said first contacting surface; wherein said secondportion is an acetabular component, said acetabular component comprisinga inner surface and wherein said inner surface comprises said secondcontacting surface. In particular embodiments of the hip implant, thediffusion hardened surface is a plasma hardened surface. In particularembodiments of the hip implant, the plasma hardened surface is selectedfrom the group consisting of a plasma carburized surface, a plasmanitrided surface and a surface both plasma carburized and plasmanitrided. In some embodiments, the medical implant is a knee implant. Inparticular knee implant embodiments, the first portion is a femoralcomponent comprising at least one condyle, said at least one condylecomprising said first contacting surface, and wherein said secondportion is a tibial component, said tibial component comprising saidsecond contacting surface. In particular embodiments of the kneeimplant, the diffusion hardened surface is a plasma hardened surface. Inparticular embodiments of the knee implant, the plasma hardened surfaceis selected from the group consisting of a plasma carburized surface, aplasma nitrided surface and a surface both plasma carburized and plasmanitrided. In other embodiments, the medical implant is selected from thegroup consisting of a shoulder implant, an elbow implant, a fingerimplant, a vertebral implant, and a tempro-mandibular implant. Inparticular embodiments, the first contacting surface and said secondcontacting surface articulate against one another. In some embodiments,the implant further comprises a ceramic coating on one or both of saidfirst and second contacting surfaces. In some embodiments, the implantfurther comprises a diamond of diamond-like carbon coating on one orboth of said first and second contacting surfaces.

In another embodiment of the present invention, there is a medicalimplant comprising: a first portion, said first portion having a firstcontacting surface; a second portion said second portion having a secondcontacting surface; wherein the first portion and the second portion arecomprised of metal or metal alloy, and wherein the first and secondcontacting surfaces contact one another; and, wherein one of conditions(a) and (b) is met: (a) one of said first and second contacting surfacescomprises a diffusion oxidized surface and the other of said first andsecond contacting surfaces is not diffusion hardened; (b) one of saidfirst and second contacting surfaces comprise a diffusion oxidizedsurface and the other of said first and second contacting surfaces isdiffusion carburized, diffusion nitrided, or both diffusion carburizedand diffusion nitrided. In some embodiments, the first contactingsurface and said second contacting surface articulate against oneanother. In particular embodiments, the metal or metal alloy is CoCr. Insome embodiments, the medical implant is a hip implant. In someembodiments, the medical implant is a knee implant. In particularembodiments, the implant further comprises a ceramic coating on one orboth of said first and second contacting surfaces. In some embodimentsthe medical implant further comprises a diamond or diamond-like carboncoating on one or both of said first and second contacting surfaces.

In another embodiment of the present invention, there is a method ofmaking a medical implant comprising the steps of: forming a medicalimplant from a metal or metal alloy, said medical implant comprising afirst portion, said first portion having a first contacting surface, anda second portion, said second portion having a second contactingsurface, wherein said first and second contacting surfaces contact oneanother; diffusion hardening said medical implant according to step (a)or (b): (a) diffusion hardening one and only one of said first andsecond contacting surfaces; (b) diffusion hardening both of said firstand second contacting surfaces wherein said first contacting surface isdiffusion hardened with a first diffusion hardening species and saidsecond contacting surface is diffusion hardened with a second diffusionhardening species and wherein said first diffusion hardening species isdifferent from said second diffusion hardening species; wherein thediffusion hardened surfaces in (a) and (b) are selected from the groupconsisting of diffusion carburized surfaces and diffusion nitridedsurfaces. In some embodiments, the first portion comprises a differentmetal or metal alloy from said second portion. In particularembodiments, the metal or metal alloy is CoCr. In particularembodiments, the step of diffusion hardening comprises plasma hardening.In some embodiments, the step of forming a medical implant comprisesforming a hip implant. In some embodiments, the step of formingcomprises forming a knee implant. In some embodiments, the step offorming a medical implant comprises forming a medical implant selectedfrom the group consisting of a shoulder implant, an elbow implant, afinger implant, a vertebral implant, and a tempro-mandibular implant. Insome embodiments, the method further comprises the step of depositing aceramic coating on one or both of said first and second contactingsurfaces. In some embodiments, the method further comprises the step ofdepositing a diamond or diamond-like carbon coating on one or both ofsaid first and second contacting surfaces.

In another embodiment of the present invention, there is a method ofmaking a medical implant comprising the steps of: forming a medicalimplant from a metal or metal alloy, said medical implant comprising afirst portion, said first portion having a first contacting surface, anda second portion, said second portion having a second contactingsurface, wherein said first and second contacting surfaces contact oneanother; diffusion hardening said medical implant according to step (a)or (b): (a) diffusion oxidizing one and only one of said first andsecond contacting surfaces and not diffusion hardening the other of saidfirst and second contacting surfaces; (b) diffusion oxidizing one ofsaid first and second contacting surfaces and diffusion hardening theother of said first and second contacting surfaces; wherein the step ofdiffusion hardening in (b) is selected from the group consisting ofdiffusion carburized surfaces and diffusion nitrided surfaces. In someembodiments, the said first portion comprises a different metal or metalalloy from said second portion. In particular embodiments, the metal ormetal alloy is CoCr. In some embodiments, the diffusion hardened surfaceis a plasma hardened surface, said diffusion oxidized surface is aplasma oxidized surface or said diffusion hardened surface is a plasmahardened surface and said diffusion oxidized surface is a plasmaoxidized surface. In some embodiments, the step of forming a medicalimplant comprises forming a hip implant. In some embodiments, the stepof forming comprises forming a knee implant. In some embodiments, thestep of forming a medical implant comprises forming a medical implantselected from the group consisting of a shoulder implant, an elbowimplant, a finger implant, a vertebral implant, and a tempro-mandibularimplant. In some embodiments, the method further comprises the step ofdepositing a ceramic coating on one or both of said first and secondcontacting surfaces. In some embodiments, the method further comprisesthe step of depositing a diamond or diamond-like carbon coating on oneor both of said first and second contacting surfaces.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims. The novel features which are believed to be characteristic ofthe invention, both as to its organization and method of operation,together with further objects and advantages will be better understoodfrom the following description when considered in connection with theaccompanying figures. It is to be expressly understood, however, thateach of the figures is provided for the purpose of illustration anddescription only and is not intended as a definition of the limits ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawing, in which:

FIG. 1 shows linear wear of as-cast and plasma carburized metal-on-metalcouples tested in a hip simulator.

FIG. 2 shows a metallographic image of carburized layer on as-cast CoCralloy sample.

FIG. 3 is a schematic diagram depicting a hip joint prosthesis in situ.

FIG. 4 is a schematic diagram showing a typical hip joint prosthesis exsitu.

FIG. 5 is a schematic diagram of a typical knee joint prosthesis insitu.

FIG. 6 is a schematic diagram showing a typical knee joint prosthesis exsitu.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, “a” or “an” means one or more. Unless otherwiseindicated or is clear by the context, the singular contains the pluraland the plural contains the singular.

As used herein, the term “articulating” as it refers to multiple medicalimplant surfaces means surfaces that contact each other in a primaryload-bearing manner. The term “contacting” as used herein as it refersto multiple medical implant surfaces refers to any surface of theimplant which contacts another surface of the implant, including bothload-bearing contact (such are articulating surfaces) and non-loadbearing contact. Thus, the term “contacting surface” is broader than,and encompasses the term “articulating surface.” The term “contacting”refers to all forms of contact and also encompasses intermediate formsof contact such as contact in secondary load bearing manner. Suchmedical implant surfaces are said to be contacting surfaces. Where animplant surface is said to be used against another implant surface itshould be understood to mean that the two surfaces are placed in aposition such that they are contacting surfaces with respect to oneanother. Contacting surfaces in a medical implant are said to be“coupled to” one another.

As used herein, “implant portion” refers to any part of a medicalimplant. Implant portions of a medical implant may be separate(detached) portions or may be separate portions of a unitary implant.

As used herein, “diffusion hardening” means the general technique ofhardening a metal or metal alloy by diffusing a hardening species intothe surface of the metal or metal alloy and into or towards the bulk ofthe metal or metal alloy. Diffusion hardening encompasses “conventionaldiffusion hardening”, whereby a diffusion hardening species in the gas,liquid or solid phase is brought into contact with a metal or metalalloy, usually at elevated temperatures, and “plasma hardening”, wherebya diffusion hardening species in a plasma is brought into contact with ametal or metal alloy to effect diffusion of the species into the metalon metal alloy. The diffusion hardening species in the present inventionare limited to carbon, nitrogen, boron, and oxygen. Thus, diffusionhardening herein results in a carburized surface, a nitrided surface, anoxidized surface, a borided surface, or any combination thereof. Herein“plasma hardened” means plasma carburized, plasma nitrided, plasmaborided, plasma oxidized or any combination thereof. A “plasma hardenedsurface” means a plasma carburized surface, a plasma nitrided surface, aplasma borided surface, a plasma oxidized surface, or any combinationthereof.

As used herein, a “diffusion hardened metallic surface” is defined asmetal or metal alloy surface has been diffusion hardened in such a wayas to either avoid formation of a ceramic layer on the surface, or toremove any such layer formed during the diffusion hardening process. A“diffusion hardened ceramic surface” is one which results when a metalor metal alloy is subject to a diffusion hardening process that resultsthe formation of a surface ceramic layer. In this way, a diffusionhardened surface differs from a diffusion hardened ceramic surface.

As used herein, CoCr alloy is defined broadly, and includes alloyshaving a least 20% (w/w) chromium and at least 50% cobalt. The balanceelements can be molybdenum tungsten, iron, nitrogen and carbon. Thetypical alloys of this composition are described in ASTM internationalstandards F799, F1537, F75 and F90 (ISO 5832-4 to ISO 5832-8. Althoughmost of the discussion focuses on CoCr alloys, it should be understoodthat the present invention is not so limited and is applicable to anymetal or metal alloy and in the context of medical implants, it ispreferable that the metal or metal alloy be a biocompatible metal ormetal alloy such as those commonly used in orthopaedic applications.

As used herein, “carburized layer” is defined is a layer of a metal ormetal alloy comprising a solid solution of the substrate metal or metalalloy and carbon. The substrate is diffused with carbon to form thesolid solution. It should be noted that it is also within the scope ofthis invention to use a nitrided layer (diffused with nitrogen), anoxidized layer (diffused with oxygen), a borided layer (diffused withboron), and combination layers using any combination of carburization,nitridation, oxidation, and boridation. Although much of the discussionherein focuses on carburization, however, it should be understood thatthe present invention includes metallic hardening with any of, or anycombination of, interstitial elements of carbon, nitrogen, oxygen, andboron. Thus, in addition to a carburized layer, one may also have anoxidized layer, a nitrided layer, a borided layer, and combinationsthereof. Although carbon, nitrogen, oxygen and boron are all useful in,and encompassed by, the present invention, carbon is most preferred andboron is least preferred. In some embodiments of the invention,therefore, the use of boron to form a borided layer is eliminated.

There has been a growing need of implants with less wear for young andactive patients. The conventional implants employ polyethylene and ametallic or ceramic counterface. Although wear of these implants issignificantly low, they do have limitations. One of the limitations issize of the implant. In order to achieve stability and greater range ofmotion for young and active patients, there has been a trend for largersized implants. This has led to emergence of metal-on-metal implants.The typical metal-on-metal implants are made of high carbon CoCr alloy.Although they offer significantly lower wear rates, the metal ionrelease from these implants and its effect on human physiology is stilla concern. As mentioned previously, it has been object of severalinventions to reduce this wear.

Conventional diffusion hardening of metals and metals alloys fororthopaedic applications is known in the art. The use ofdiffusion-hardened oxide surfaces such as oxidized zirconium inorthopedic applications was first demonstrated by Davidson in U.S. Pat.No. 5,037,438. The Davidson '438 patent discloses a method of producingzirconium alloy prostheses with an oxidized zirconium surface. U.S. Pat.No. 2,987,352 to Watson discloses a method of producing zirconiumbearings with a oxidized zirconium surface. The oxide coating producedis not always uniform in thickness and the non-uniformity reduces theintegrity of the bonding between the zirconium alloy and the oxide layerand the integrity of the bonding within the oxide layer. Davidson andWatson both teach diffusion hardening of zirconium alloys. In thegeneral method, a zirconium alloy is heated in air at elevatedtemperature. Heating in air leads to diffusion of oxygen in thesubstrate and thus forms the ceramic oxide on the surface. In anotherapproach Shetty et. al. (U.S. Pat. No. 5,308,412) teach nitrogendiffusion hardening of CoCr alloys. Shetty teach heating CoCr alloy innitrogen enriched environment (ionic or molecular) for prolonged periodsof time to diffusion harden the surface with nitrogen. U.S. Pat. No.5,152,794 to Davidson extends the concept of conventional diffusionhardening to diffusion nitriding (using a nitrogen source for thenitridation). U.S. Pat. Nos. 2,987,352; 5,037,438; 5,308,412; and5,152,794 are incorporated by reference as though fully set forthherein.

In U.S. Pat. Nos. 6,447,550; 6,585,772 and pending U.S. application Ser.No. 10/942,464, Hunter, et al. describes methods for obtaining anoxidized zirconium coating of uniform thickness. Hunter teaches thatsuch is obtained by applying pre-oxidation treatment techniques and bymanipulation of substrate microstructure. The use of uniform thicknessoxide layer results in increased resistance to corrosion by the actionof the body fluids as well as other benefits and is biocompatible andstable over the lifetime of the recipient. U.S. Pat. Nos. 6,447,550;6,585,772 and pending U.S. application Ser. No. 10/942,464 areincorporated by reference as though fully set forth herein. Hunter et.al. teach two approaches to obtain a uniform oxide. One approach is toachieve an adequate surface roughness as pre-oxidation step. Thissurface roughness allows diffusion of oxygen uniformly to create anadherent and uniform oxide on the surface. In another approach Hunterteach use of a refined fined grained microstructure. The purpose here isalso to allow diffusion of oxygen uniformly in to the substrate and thuslead to uniform adherent oxide. However, it should be understood thatthe compositions of Davidson, Watson, and Hunter were diffusion hardenedceramic surfaces. The surfaces of the present differ in that they arediffusion hardened metallic surfaces which is explained below.

The conventional diffusion hardening processes of Davidson and Hunterallow interstitial elements (such as C, N, O, and/or B) to diffuse in tothe metallic surface. As these elements diffuse in the surface it maysaturate the metal surface and thus lead to hardening of the surface. Ifthe diffusion is continued further beyond the solubility limit of theinterstitial element in the substrate it can lead to formation of aceramic surface or a compound formation along with a diffusion hardenedzone. An example of which is Davidson and Hunter process that leads toformation of the oxide on Zr alloy surface. It is the object of thisinvention to only form diffusion hardened metallic surface and not forma ceramic layer or compound on the surface. If the ceramic layer orcompound is formed during the conventional process, it can be removed byknown techniques in the art. It is known to those skilled in the artthat a diffusion process can be carried out by several means. Theconventional diffusion hardening processes that are known in the art arepack carburizing or nitriding, salt bath carburizing or nitriding orusing gaseous mixtures of diffusing species. For example, a device madefrom CoCr alloy is heated in presence of 100% nitrogen gas attemperature ranging from 250° C. to 1000° C. for extended periods oftime. The longer diffusion times will create thicker diffusion hardeningzone. The typical time of hardening can be 5 hours or more. The devicecan be placed in a natural convection oven or can alternatively beheated by resistive heating. After such prolonged treatment a diffusionhardened zone is formed along with a thin ceramic layer (chromiumnitride) on the surface. This ceramic layer is now removed by polishingwith grinding medium such as but not limited to silicon carbides oralumina or silica. It is the object of this invention to create asurface that is diffusion hardened and is metallic in nature Anotherprocesses in the art for diffusion hardening is plasma hardening. Itshould be understood by those skilled in the art that the objective isto have the presence of diffusion specie either in the solid or liquidor gaseous or plasma forms or combinations thereof. For example,nitrogen diffusion hardening of Ti alloys can be carried out in anitrogen rich atmosphere or in nitrogen gas plasma. If nitrogendiffusion is carried out for long times of titanium nitrides on thesurface. While conventional diffusion hardening sometimes results in aceramic coating on the surface, it is the object of this invention toallow the saturation only to the extent that the ceramic layer is notformed or if it is formed it is removed by mechanically, or by heatingthe surface in the absence of a diffusion hardening species such thatdiffusion hardening species is driven deeper into the substrate, therebyconsuming any ceramic layer which may have formed. The examples ofmechanical removal of the ceramic layer include grinding with abrasivemedia such as silicon carbide, alumina or silica. Although most of theceramic surfaces will be difficult to remove by acids or alkalies,chemical-mechanical polishing methods can be used. The chemicalpolishing methods employ abrasive media suspended in acidic or alkalinemedium which will enhance removal rates of ceramic surface.Alternatively, formation of a ceramic layer in conventional diffusionhardening processes may be avoided by judicious control of the processvariables (time, temperature, concentration of diffusion hardeningspecies, etc.) avoid ceramic layer formation. It may be necessary tomodify the variables of the conventional diffusion hardening processdepending upon the nature of the metal or metal alloy substrate, thenature of the diffusion hardening species, and possibly other factors.In some cases, it may be necessary to run a sample under a set ofconditions, then perform analytical tests on the sample to determine thepresence of and the extent of ceramic layer formation. The presence ofceramic layer can be determined by conventional cross-sectionalmetallographic techniques. It can also be determined from the chemicalcomposition of the surface and based on the solubility of diffusingspecie in that alloy system. The solubility of the diffusing speciein-turn can be obtained from the phase diagrams. The presence of ceramiclayer can also be determined using a capacitance-voltage measurement ofthe sample. Since a ceramic layer is typically a insulator a ceramiclayer on the surface of a metal creates a capacitor. Alternativelyceramic layer or its thickness can be measured using depth profiling andx-ray photoelectron spectroscopy (XPS). In XPS, the valence state of thediffusing specie can be determined which in turn can help in estimatingthe depth of the ceramic layer. Other analytical techniques,particularly surface analysis techniques, known to those of ordinaryskill in the art are applicable. Process variables can then be modifiedto preclude formation of the ceramic. Similarly, when removing a ceramicby mechanical, chemical, or heat treatment, one may perform analyticaltests on the sample to determine the extent of ceramic layer removal. Inthe present invention, wherein a diffusion hardened surface is formedusing that conventional diffusion hardening processes of Davidson,Watson, and Hunter, it is required that the final product is a diffusionhardened metallic surface and not a diffusion hardened ceramic surface.

As discussed, another mechanism for diffusion hardening of metals andmetal alloys to produce a diffusion hardened metallic surface comprisesthe surface modification of these materials by the incorporation ofnon-metallic species by treatment of the metal or metal alloy with suchspecies in a plasma. A plasma is typically an ionized gas (atoms ormolecules), and is usually considered to be a distinct phase of matterin contrast to solids, liquids, and gases because of its uniqueproperties. The ionization of the gaseous atoms or molecules results inat least one electron being dissociated from a proportion of the atomsor molecules. The resulting free electric charges make the plasmaelectrically conductive so that it responds strongly to electromagneticfields. Plasmas may be created by electromagnetic fields (e.g., RF ormicrowave plasmas) or by voltage discharges (e.g., glow discharge). Insome cases, like that discussed for conventional diffusion hardening, itmay be necessary to either avoid ceramic layer formation or remove anysuch layer. This is analogous to the situation discussed forconventional diffusion hardening. Removal techniques are the same asthose for samples which are diffusion hardened conventionally. Alsosimilar to conventional diffusion hardening, it may be necessary tomodify the variables of the plasma hardening process depending upon thenature of the metal or metal alloy substrate, the nature of thediffusion hardening species, and possibly other factors. In some cases,it may be necessary to run a sample under a set of conditions, thenperform analytical tests on the sample to determine the presence of andthe extent of ceramic layer formation.

Plasma hardening of CoCr alloy using carbon has been describedpreviously by Bell et. al (U.S. Patent Application No. 2005/0241736 A1).Diffusion hardening using nitrogen has been taught by Shetty et. al.(U.S. Pat. No. 5,308,412). The '736 patent application describes thecarburization process. It further states that such processed implantscan be used in knee or hip joints. The '412 patent describes thenitridation process. It should be noted that both these inventions donot anticipate the coupling of such a hardened implant to a non-hardenedimplant. U.S. Patent Application No. 2005/0241736 A1 and U.S. Pat. No.5,308,412 are incorporated by reference as though fully set forthherein.

However, surface hardening by carbuziation and/or nitridation and/oroxidation and/or boridation can be performed by any and all methodsknown in the art. The preferred embodiments use the conventionaldiffusion hardening or plasma hardening techniques discussed above. Thepresent invention includes carburization, nitridation, boridation,oxidation, and combinations thereof, of surfaces of metallic devicesgenerally, and of medical implants specifically, particularly, thosesurfaces of medical implants and metallic devices subject to wear suchas articualting surfaces.

Generally, plasma treatment in the context of the present inventionmeans exposing a metal or metal alloy to a plasma in the presence of acarbon source, an oxygen source, a nitrogen source, a boron source, orany combination thereof. Examples of a carbon source include methane;examples of a nitrogen source includes nitrogen gas; examples of anoxygen source include oxygen gas and air. The examples of boron sourceinclude but not limited to amorphous boron, boric acid (B₂O₃), and borontrichloride (BCl₃). These are merely illustrative examples of sources ofdiffusion species, others known to those of skill in the art are alsoapplicable. The same sources are useful in conventional diffusionhardening techniques.

One exemplary, non-limiting procedure is provided for the plasmacarburization of a medical implant. An implant is placed on a surfaceinside a vessel, the surface being connected as a cathode to a powersupply and control unit, and the wall of the vessel is connected to thedirect current source as the anode. The temperature of the implant wasmeasured by the thermocouple inserted into a hole of 3 mm diameterdrilled in an implant portion or a dummy sample. After the sealablevessel is tightly closed, a rotary pump is used to remove the residualair (oxygen) and thus reduce the pressure in the vessel. When thereduction in pressure reaches 10 Pa (0.1 mbar) or less, a glow dischargewas introduced between the implant and the vessel wall (anode) byapplying a voltage of about 400 volts to about 900 volts between thesetwo electrodes. A heating gas of hydrogen was at the same timeintroduced into the vessel. The pressure of the hydrogen gas in thevessel is increased gradually as the temperature of the implantincreases. No external or auxiliary heating is employed, and the implantis heated by the glow discharge plasma only. Alternatively, an externalheat source may be used.

In other embodiments, an external heater attached to the vessel may beemployed, or a combination of external heating and electrical glowdischarge heating may be employed. Direct current (dc) discharge, pulseddc discharge or alternating current (ac) discharge may be used.

After the implant is heated to the prescribed temperature, a gas mixtureof hydrogen (98.5%) and methane (1.5%) is introduced into the vessel andthe plasma treatment started. Treatment temperatures from about 400° C.to about 600° C. are employed for a treatment time of 10 hours. Theworking pressure in the treatment step is 500 Pa (5.0 mbar).

During the plasma heat treatment, the methane is ionized, activated anddissociated to produce carbon ions and activated carbon atoms andneutral molecules, which then diffuse into the surface of the discforming a carbon diffusion layer. When the plasma treatment is carriedout at a relatively low temperature ranging from 300 to 550° C., thecarbon atoms mainly reside in the cobalt lattices, forming asupersaturated solid solution with a possible nanocrystalline structuredue to the relatively low temperatures employed in the treatment. Theresultant layer has a high hardness, good fatigue strength and excellentwear and corrosion resistance (see below). When the plasma treatment iscarried out at a relatively high temperature ranging from 600 to 700°C., the carbon atoms partially reside in the cobalt lattices forming asupersaturated solid solution and partially combined with carbon formingchromium carbides. The resultant layer has a high hardness, fatiguestrength and excellent wear resistance.

After the completion of the plasma treatment, the glow discharge plasmais turned off and the implant is allowed to cool in the vessel in thetreatment atmosphere down to room temperature before they were removedfrom the vessel. Treatment times and temperatures may be varied toachieve the desired level of nitridation. For nitridation of theimplant, the same procedure can be used with a gas mixture of hydrogenand nitrogen, or nitrogen alone. Again, treatment times and temperaturesmay be varied to achieve the desired level of nitridation. Finally, itis possible to use both the nitrogen and carbon source (for example amixture of methane, hydrogen and nitrogen, etc.) for both carburize andnitride. Alternatively, separate carburization and nitridation steps maybe performed sequentially on the same implant portion.

It should be understood that the example provided above is merelyillustrative and not exhaustive; any known or yet to be developed plasmanitridation and/or carburization procedures may be used. All that isrequired is that a sufficient degree of carburization and/or nitridationof a metal or metal alloy (preferably CoCr) surface is applicable in thepresent invention. In some embodiments of the invention, it ispreferential to form carburized or nitrided layer at least 1 micronthick.

The inventors have unexpectedly found that the wear performance of firstand second contacting metallic surfaces of medical implants,(particularly where the contacting surfaces are articulating surfaces)is significantly lower, when the first contacting surface is a diffusionhardened metallic surface and the second contacting surface is either 1)not diffusion hardened or 2) diffusion hardened using a hardeningspecies different from that used in the first contacting surface. In thepresent invention, where a surface is diffusion hardened, it isdiffusion hardened to form a diffusion hardened metallic surface. Thereason for the lower wear is not completely clear. Although not wishingto be bound by theory, it is possible that the differential hardness ofthe contacting surfaces (owing to the differing treatment of thesurfaces) and/or the different compositions of the first and secondcontacting surfaces may play a role in the beneficial effect seen. Ithas been observed that a plasma carburized CoCr surface contacting anon-carburized CoCr surface in a load bearing manner exhibits superiorwear characteristics when compared to a non-plasma carburized CoCrsurface against another non-plasma carburized CoCr surface, and also ascompared to a plasma carburized CoCr surface against another plasmacarburized CoCr surface.

FIG. 1 shows linear wear of as-cast and plasma carburized CoCrmetal-on-metal couples tested in a hip simulator. The linear wear ofas-cast articulating against as-cast metal-on-metal couple is 7 micronat the end of 0.5 Mcycles. It increases to 10 microns at the end of 1Mcycle. It then reaches a steady state and stabilizes at 10 microns. Theinventors have found that this wear rate can be significantly reduced ifonly one of the mating couple is selectively hardened for example, usingcarburization process. As shown in FIG. 2, the linear wear when thefemoral head is carburized and acetabular shell is not carburized isapproximately 3 microns and stabilizes at 5 microns up to 2.5 Mcycle.Thus leads to approximately 50% reduction in linear wear. As shown inFIG. 1, the inventors have found that when both couples are selectivelyhardened, the wear is actually more than when they are not hardened. Thelinear wear of both components that are carburized is approximately 6microns at 0.5 Mcycle. It increases to 23 microns at the end of 1 Mcycleand reaches approximately 35 microns at the end of 2.5 Mcycles. Thereduction in wear of the mating couple by selectively hardening onecomponent of the couple is a particular embodiment of this invention. Itwill be understood by those skilled in the art that similar results canbe obtained if one of the couple had been nitrided.

In an embodiment of the invention, the substrate alloy that is beingcarburized can be as-cast. FIG. 2 shows a metallographic image of thecarburized layer of as-cast component. The carburized layer isapproximately 8 micron thick. As a preferred embodiment of thisinvention, the carburized component is coupled with non-carburizedcomponent. The non-carburized component can be as-cast or is machinedfrom a wrought bar stock. Alternatively, it can be made from a forgedcomponent. It should be noted that a particular embodiment of thisinvention is to take advantage of the differences in the surfacehardness/compositional characteristics of the mating components.

Alternatively, the substrate alloy that is being carburized is machinedfrom wrought bar stock or is forged. This carburized component is thencoupled with non-carburized component. The non-carburized component canbe as-cast or is machined from a wrought bar stock. Alternatively, itcan be made from a forged component.

In an embodiment of invention, the carburized layer thickness is from 1to 25 microns. In as-cast components, the carburized layer may encompassthe substrate carbides as shown in FIG. 2. In wrought components, thecarburized layer may or may not encompass fine carbides based on thepre-carburization heat treatment of the substrate alloy. In one aspectof invention the carburized layer is approximately 1.2 times harder thanthe substrate alloy.

Although the example of FIG. 1 showed the unexpected improvement in wearperformance that is realized in using plasma carburized CoCr surfaceagainst an untreated (as cast) CoCr surface, the present invention isnot so limited. Although not wishing to be bound by theory, it issuspected that the improvement is based upon the differential hardnessbetween the plasma carburized CoCr surface and the untreated (as cast)CoCr surface. Accordingly, it is suspected that the use of differentialtreatment of cooperating surfaces will result in wear performanceimprovement in any hard-on-hard couple. For example, it is also withinthe scope of the present invention to have a medical implant havingplasma carburized CoCr surface cooperating against a plasma nitridedCoCr surface. Because two different diffusion species are used, adifferential hardness in the cooperating surfaces will result. Othercombinations of plasma hardened against another plasma hardened surface(hardened with a different species) are also within the scope of thepresent invention. For example, one may use a plasma oxidized CoCragainst a plasma nitrided CoCr, etc. For plasma hardening, any of plasmacarburization, plasma nitridation, and plasma oxidation are useful. Eachcan be used against a plasma hardened surface (hardened with one of theother diffusion hardening species) or alternatively, they can be usedagainst an untreated CoCr surface. Finally, any one of a plasmacarburized surface, a plasma nitrided surface, or a plasma oxidizedsurface may be used against a surface hardened with a conventionaldiffusion hardening technique. One example of this would be any of aplasma carburized surface, a plasma nitrided surface, or a plasmaoxidized surface used against a Davidson-type diffusion hardenedsurface.

In some embodiments of the invention the diffusion hardened layer isused as a pre-treatment surface for deposition of ceramic coatings. Itis important to draw a distinction between these embodiments (depositedceramic coatings) and the in-situ ceramic coatings that can form duringany diffusion hardening processes (both conventional diffusion hardeningprocess and plasma hardening). The ceramic coatings can be oxides,nitrides, carbides, or borides. Examples of such coating includealuminum oxide, aluminum nitride, chromium carbide, chromium nitride,titanium carbide, titanium nitride and titanium carbonitride. Suchcoatings can be deposited by methods known in the art, such as, but notlimited to physical vapor deposition (PVD) or chemical vapor deposition(CVD). The ceramic coated component is then coupled with a diffusionhardened or a non-diffusion hardened component in a medical implant.Alternatively, the hard ceramic coating is performed on a non-diffusionhardened component which is coupled with a diffusion hardened component.

In other embodiments of the invention, the diffusion hardened componentis used as a surface for diamond or diamond like carbon coatings (e.g.,nanocrystalline diamond) which is coupled with non-diffusion hardenedcomponent. Alternatively, the diffusion hardened component is coatedwith diamond or diamond like carbon coating which is coupled with adiffusion hardened component. Again the coatings of these embodiments,unlike ceramic coatings which may form in either a conventionaldiffusion hardening process or in a plasma hardening process, are notformed in-situ, but are rather deposited or otherwise placed on thepreviously formed diffusion hardened metallic surface.

In an embodiment of the invention, antimicrobial surface is created onthe carburized surface. This is accomplished by impregnating carburizedlayer with silver ions. The silver ions can be impregnated with ionimplantation or using diffusion processes. The silver impregnatedcarburized component is then coupled with non-carburized or ceramiccoated carburized component.

The present invention also provides a low friction, wear resistantsurface on the articulating surfaces of CoCr prosthetic devices.Illustrative examples of such articulating surfaces are shown in theschematic diagrams of FIGS. 3-6 which illustrate hip and knee prostheticdevices. It should be understood that FIGS. 3-6 merely representillustrative examples and that the present invention is applicable to awide variety of prosthetic implants. Other examples include shoulderimplants, elbow implants, finger implants, vertebral implants, and atempro-mandibular implants.

A typical hip joint assembly is shown in situ in FIG. 3 and ex-situ inFIG. 4. The hip joint stem 2 fits into the femur while the femoral head6 of the prosthesis fits into and articulates against the inner surface8 of an acetabular cup 10 which in turn is affixed to the pelvis asshown in FIGS. 3 and 4. A porous metal bead or wire mesh coating 12 maybe incorporated to allow stabilization of the implant by ingrowth ofsurrounding tissue into the porous coating. The femoral head 6 may be anintegral part of the hip joint stem 2 or may be a separate componentmounted upon a conical taper at the end of the neck 4 of the hip jointprosthesis. In this embodiment of a hip implant, the femoral headarticulates against the inner surface of the acetabular cup therebycausing wear and, in the long term, this may necessitate prosthesisreplacement. The inner surface can be in the form of a liner that fitsin acetabular cup or can be an integral part of the acetabularcomponent. In a preferred embodiment of the present invention, both thefemoral head 6 and the acetabular cup 10 are made of CoCr alloy, whereasthe surfaces of one and only one of either the femoral head 6 or theinner surface 8 of the acetabular cup 10 is diffusion hardened. Thisarticulating couple exhibits unexpectedly low wear rates as compared toa CoCr surface (non-diffusion hardened) articulating against anotherCoCr surface (non-diffusion hardened) or a diffusion hardened CoCrsurface articulating against another diffusion hardened CoCr surface. Inother embodiments, both articulating surfaces are diffusion hardened,with the provision that one of the surfaces is diffusion hardened with adiffusion species which is not common to the other surface. For example,there may be a carbonitrided surface articulating against a carburizedsurface. In preferred embodiments, the diffusion hardening isaccomplished through plasma hardening, preferably plasma carburization,although convention diffusion hardening and combinations of plasmahardening and diffusion hardening are applicable. Other diffusionhardening techniques known in the art are also applicable, both alone orin combination with plasma hardening and diffusion hardening. Theembodiment of FIGS. 3 and 4 illustrate total hip arthoplasty. However,it should be understood that the present invention is applicable notonly to total hip arthoplasty but also to resurfacing hip arthoplasty,such as but not limited to what is commercially known as Birmingham HipResurfacing. Resurfacing hip arthoplasty conserves bone by eliminatingthe large femoral stem of the total hip implant.

A typical knee joint prosthesis is shown in situ in FIG. 5 and ex-situin FIG. 6. The knee joint includes a femoral component 20 and a tibialcomponent 30. The femoral component includes condyles 22 which providethe articulating surface of the femoral component and pegs 24 foraffixing the femoral component to the femur. The tibial component 30includes a tibial base 32 with a peg 34 for mounting the tibial baseonto the tibia. A tibial platform 36 is mounted atop the tibial base 32and is supplied with grooves 38 similar to the shape of the condyles 22.The bottom surfaces of the condyles 26 contact the tibial platform'sgrooves 38 so that the condyles articulate within these grooves againstthe tibial platform. As in the case of the hip joint, porous bead orwire mesh coatings can also be applied to either the tibial or femoralcomponents of the knee or both. In a preferred embodiment of the presentinvention, both the femoral component 20 and the tibial component 30 aremade of CoCr alloy, whereas one and only one of either the femoralcomponent 20 and the tibial component 30 is diffusion hardened. Again,this articulating couple will exhibit unexpectedly low wear rates ascompared to a CoCr surface (non-diffusion hardened) articulating againstanother CoCr surface (non-diffusion hardened) or a diffusion hardenedCoCr surface articulating against another diffusion hardened CoCrsurface. In other embodiments, both articulating surfaces are diffusionhardened, with the provision that one of the surfaces is diffusionhardened with a diffusion species which is not common to the othersurface. For example, there may be a carbonitrided surface articulatingagainst a carburized surface. In preferred embodiments, the diffusionhardening is accomplished through plasma hardening, preferably plasmacarburization, although convention diffusion hardening and combinationsof plasma hardening and diffusion hardening are applicable. Otherdiffusion hardening techniques known in the art are also applicable,both alone or in combination with plasma hardening and diffusionhardening.

It should be understood that although the preferred substrate in theimplants and methods of the present invention is CoCr, the use of anymetal or metal alloy substrate is within the scope of the presentinvention. For medical implant applications, such metals or metal alloysare preferably bio-compatible. Broadly, the scope extends to an implanthaving contacting surfaces of a metal or metal alloy, wherein one of thesurfaces is diffusion hardened and the other contacting surface iseither not diffusion hardened or is diffusion hardened with a diffusionhardened species not used in the opposing contacting surface.

Because of the superior wear characteristics of the metallic couple ofthe present invention, it has application is areas outside of medicalimplants. For example, the present invention is applicable in anyapplications wherein metallic components bear against one another.Non-limiting examples include parts and components (and devices ingeneral) in the automotive and aerospace industries. Other commercialand industrial components are also applicable to the present inventionand are encompassed by it. Other bearing applications would also benefitfrom an improved couple It should be clear to one of ordinary skill inthe art than other applications are encompassed herein, so long as thereis involved a metallic-based couple that is subject to wear.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. A method of making an orthopedic implantcomprising the steps of: forming an orthopedic implant comprising one ormore first components and one or more second components, said formingstep comprises forming said one or more first components from a firstmetal or metal alloy, each of said one or more first components having afirst articulating surface: and forming said one or more secondcomponents from a second metal or metal alloy, each of said one or moresecond components having a second articulating surface, wherein any ofsaid first articulating surface of said one or more first components isconfigured to articulate against any of said second articulating surfaceof said one or more second components; and forming a diffusion hardenedmetallic surface on said orthopedic implant by diffusion hardening, saidstep of diffusion hardening being performed with a diffusion hardeningspecies selected from the group consisting of carbon, nitrogen, oxygen,boron, and any combination thereof, said step of diffusion hardeningperformed according to step (a) or (b): (a) diffusion hardening to forma diffusion hardened metallic surface on one and only one of said firstand second articulating surfaces, wherein the non-diffusion hardenedsurface comprises a metal or metal alloy surface; (b) diffusionhardening to form a diffusion hardened metallic surface on both of saidfirst and second articulating surfaces wherein said first articulatingsurface is diffusion hardened with at least a first diffusion hardeningspecies and said second articulating surface is diffusion hardened withat least a second diffusion hardening species and wherein said firstdiffusion hardening species is different from said second diffusionhardening species.
 2. The method of claim 1, wherein said first metal ormetal alloy is different from said second metal or metal alloy.
 3. Themethod of claim 1, wherein said first and second metals or metal alloysare cobalt chrome.
 4. The method of claim 1, wherein said step ofdiffusion hardening comprises plasma hardening.
 5. The method of claim1, wherein said orthopedic implant comprises a hip implant.
 6. Themethod of claim 5, wherein one of said one or more first components andsaid one or more said second components is an acetabular component andthe other of said one or more first components and said one or moresecond components is a femoral component.
 7. The method of claim 1,further comprising the step of depositing a ceramic coating on one orboth of said first and second articulating surfaces.
 8. The method ofclaim 1, further comprising the step of depositing a diamond ordiamond-like carbon coating on one or both of said first and secondarticulating surfaces.
 9. The method of claim 1, wherein said step ofdiffusion hardening to form a diffusion hardened metallic surface onboth of said first and second articulating surfaces comprises forming adiffusion carburized and diffusion oxidized surface on said firstarticulating surface and forming a diffusion carburized surface on saidsecond articulating surface.
 10. The method of claim 1, wherein step (b)applies and, wherein one of said articulating surfaces is diffusionhardened with a diffusion hardening species that is not present as adiffusion hardening species in the other articulating surface.
 11. Amethod of making an orthopedic implant comprising the steps of: formingan orthopedic implant comprising a one or more first components and aone or more second components, said step comprises forming said one ormore first components from a first metal or metal alloy, each of saidone or more first components having a first articulating surface;forming said one or more second components from a second metal or metalalloy, each of said one or more second components having a secondarticulating surface, wherein any of said first articulating surface ofsaid one or more first components is configured to articulate againstany of said second articulating surface of said one or more secondcomponents; and forming a diffusion hardened metallic surface on one andonly one of said first and second articulating surfaces, wherein thenon-diffusion hardened surface comprises a metal or metal alloy surface.12. The method of claim 11, wherein said first metal or metal alloy isdifferent from said second metal or metal alloy.
 13. The method of claim11, wherein said first and second metals or metal alloys are cobaltchrome.
 14. The method of claim 11, wherein said step of forming adiffusion hardened metallic surface comprises plasma hardening.
 15. Themethod of claim 11, wherein said orthopedic implant comprises forming ahip implant.
 16. The method of claim 15, wherein one of said one or morefirst components and said one or more second components is an acetabularcomponent and the other of said one or more first components and saidone or more second components is a femoral component.
 17. The method ofclaim 11, further comprising the step of depositing a ceramic coating onone or both of said first and second articulating surfaces.
 18. Themethod of claim 11, further comprising the step of depositing a diamondor diamond-like carbon coating on one or both of said first and secondarticulating surfaces.
 19. The method of claim 11, wherein said step offorming a diffusion hardened metallic surface comprises diffusionhardening performed with a diffusion hardening species selected from thegroup consisting of carbon, nitrogen, oxygen, and any combinationthereof.
 20. A method of making an orthopedic implant comprising thesteps of: forming an orthopedic implant comprising a one or more firstcomponents and a one or more second components, said forming stepcomprises forming said one or more first components from a first metalor metal alloy, each of said one or more first components having a firstarticulating surface; forming said one or more second components from asecond metal or metal alloy, each of said one or more second componentshaving a second articulating surface; wherein any of said firstarticulating surface of said one or more first components is configuredto articulate against any of said second articulating surface of saidone or more second components; forming a diffusion hardened metallicsurface on said first articulating surface with a first diffusionhardening species; and forming a diffusion hardened metallic surface onsaid second articulating surface with a second hardening species,wherein said first diffusion hardening species is different from saidsecond diffusion hardening species.
 21. The method of claim 20, whereinsaid first metal or metal alloy is different from said second metal ormetal alloy.
 22. The method of claim 20, wherein said first and secondmetals or metal alloys are cobalt chrome.
 23. The method of claim 20,wherein said step of forming a diffusion hardened metallic surfacecomprises plasma hardening.
 24. The method of claim 20, wherein saidorthopedic implant comprises a hip implant.
 25. The method of claim 24,wherein one of said one or more first components and said one or moresecond components is an acetabular component and the other of said oneor more first components and said one or more second components is afemoral component.
 26. The method of claim 20, further comprising thestep of depositing a ceramic coating on one or both of said first andsecond articulating surfaces.
 27. The method of claim 20, furthercomprising the step of depositing a diamond or diamond-like carboncoating on one or both of said first and second articulating surfaces.28. The method of claim 20, wherein said step of forming a diffusionhardened metallic surface on said first articulating surface and saidsecond articulating surface comprises forming a diffusion oxidizedsurface on said first articulating surface and forming a diffusioncarburized surface on said second articulating surface.
 29. The methodof claim 20, wherein said step of forming a diffusion hardened metallicsurface on said first articulating surface comprises diffusion hardeningperformed with a diffusion hardening species selected from the groupconsisting of carbon, nitrogen, boron, and any combination thereof; and,forming a diffusion hardened metallic surface on said secondarticulating surface comprises diffusion hardening performed with adiffusion hardening species selected from the group consisting ofcarbon, nitrogen, oxygen, boron, and any combination thereof, subject tothe condition that one of said articulating surfaces is diffusionhardened with a diffusion hardening species that is not present as adiffusion hardening species in the other articulating surface.
 30. Themethod of claim 20, wherein said step of diffusion hardening to form adiffusion hardened metallic surface on both of said first and secondarticulating surfaces comprises forming a diffusion carburized anddiffusion oxidized surface on said first articulating surface andforming a diffusion carburized surface on said second articulatingsurface.